added system.default in order to prevent the abstraction inversion that 'template default(T): untyped = (var x: T; x)' causes

changelog.md

@@ -130,6 +130,7 @@ proc enumToString*(enums: openArray[enum]): string =
 
 - Added the `posix_utils` module.
 
+- Added `system.default`.
 
 ### Library changes
 

compiler/ast.nim

@@ -635,7 +635,7 @@ type
     mSwap, mIsNil, mArrToSeq, mCopyStr, mCopyStrLast,
     mNewString, mNewStringOfCap, mParseBiggestFloat,
     mMove, mWasMoved, mDestroy,
-    mReset,
+    mDefault, mReset,
     mArray, mOpenArray, mRange, mSet, mSeq, mOpt, mVarargs,
     mRef, mPtr, mVar, mDistinct, mVoid, mTuple,
     mOrdinal,

compiler/ccgexprs.nim

@@ -1153,6 +1153,9 @@ proc genReset(p: BProc, n: PNode) =
           addrLoc(p.config, a),
           genTypeInfo(p.module, skipTypes(a.t, {tyVar}), n.info))
 
+proc genDefault(p: BProc; n: PNode; d: var TLoc) =
+  resetLoc(p, d)
+
 proc rawGenNew(p: BProc, a: TLoc, sizeExpr: Rope) =
   var sizeExpr = sizeExpr
   let typ = a.t
@@ -2063,6 +2066,7 @@ proc genMagicExpr(p: BProc, e: PNode, d: var TLoc, op: TMagic) =
         addf(p.module.s[cfsDynLibInit], "\t$1 = ($2) hcrGetProc($3, \"$1\");$n",
              [mangleDynLibProc(prc), getTypeDesc(p.module, prc.loc.t), getModuleDllPath(p.module, prc)])
     genCall(p, e, d)
+  of mDefault: genDefault(p, e, d)
   of mReset: genReset(p, e)
   of mEcho: genEcho(p, e[1].skipConv)
   of mArrToSeq: genArrToSeq(p, e, d)

compiler/condsyms.nim

@@ -85,6 +85,7 @@ proc initDefines*(symbols: StringTableRef) =
   defineSymbol("nimHasHotCodeReloading")
   defineSymbol("nimHasNilSeqs")
   defineSymbol("nimHasSignatureHashInMacro")
+  defineSymbol("nimHasDefault")
   for f in low(Feature)..high(Feature):
     defineSymbol("nimHas" & $f)
 

compiler/vmgen.nim

@@ -910,6 +910,15 @@ proc genBindSym(c: PCtx; n: PNode; dest: var TDest) =
     c.gABC(n, opcNDynBindSym, dest, x, n.len)
     c.freeTempRange(x, n.len)
 
+proc fitsRegister*(t: PType): bool =
+  assert t != nil
+  t.skipTypes(abstractInst-{tyTypeDesc}).kind in {
+    tyRange, tyEnum, tyBool, tyInt..tyUInt64, tyChar}
+
+proc ldNullOpcode(t: PType): TOpcode =
+  assert t != nil
+  if fitsRegister(t): opcLdNullReg else: opcLdNull
+
 proc genMagic(c: PCtx; n: PNode; dest: var TDest; m: TMagic) =
   case m
   of mAnd: c.genAndOr(n, opcFJmp, dest)
@@ -1129,9 +1138,13 @@ proc genMagic(c: PCtx; n: PNode; dest: var TDest; m: TMagic) =
   of mReset:
     unused(c, n, dest)
     var d = c.genx(n.sons[1])
+    # XXX use ldNullOpcode() here?
     c.gABx(n, opcLdNull, d, c.genType(n.sons[1].typ))
     c.gABx(n, opcNodeToReg, d, d)
     c.genAsgnPatch(n.sons[1], d)
+  of mDefault:
+    if dest < 0: dest = c.getTemp(n.typ)
+    c.gABx(n, ldNullOpcode(n.typ), dest, c.genType(n.typ))
   of mOf, mIs:
     if dest < 0: dest = c.getTemp(n.typ)
     var tmp = c.genx(n.sons[1])
@@ -1332,11 +1345,6 @@ const
     tyFloat, tyFloat32, tyFloat64, tyFloat128,
     tyUInt, tyUInt8, tyUInt16, tyUInt32, tyUInt64}
 
-proc fitsRegister*(t: PType): bool =
-  assert t != nil
-  t.skipTypes(abstractInst-{tyTypeDesc}).kind in {
-    tyRange, tyEnum, tyBool, tyInt..tyUInt64, tyChar}
-
 proc unneededIndirection(n: PNode): bool =
   n.typ.skipTypes(abstractInstOwned-{tyTypeDesc}).kind == tyRef
 
@@ -1766,10 +1774,6 @@ proc getNullValue(typ: PType, info: TLineInfo; conf: ConfigRef): PNode =
     globalError(conf, info, "cannot create null element for: " & $t.kind)
     result = newNodeI(nkEmpty, info)
 
-proc ldNullOpcode(t: PType): TOpcode =
-  assert t != nil
-  if fitsRegister(t): opcLdNullReg else: opcLdNull
-
 proc genVarSection(c: PCtx; n: PNode) =
   for a in n:
     if a.kind == nkCommentStmt: continue

lib/system.nim

@@ -4278,6 +4278,10 @@ proc `$`*(t: typedesc): string {.magic: "TypeTrait".} =
     doAssert $(type("Foo")) == "string"
     static: doAssert $(type(@['A', 'B'])) == "seq[char]"
 
+when defined(nimHasDefault):
+  proc default*(T: typedesc): T {.magic: "Default", noSideEffect.}
+    ## returns the default value of the type ``T``.
+
 import system/widestrs
 export widestrs
 

tests/vm/treset.nim

@@ -1,3 +1,6 @@
+discard """
+  output: '''0'''
+"""
 static:
   type Obj = object
     field: int
@@ -5,6 +8,17 @@ static:
   reset(o)
   doAssert o.field == 0
 
+  var x = 4
+  reset(x)
+  doAssert x == 0
+
+static:
+  type ObjB = object
+    field: int
+  var o = ObjB(field: 1)
+  o = default(ObjB)
+  doAssert o.field == 0
+
 static:
   var i = 2
   reset(i)
@@ -25,4 +39,15 @@ static:
     var i = 2
     reset(i)
     doAssert i == 0
-  f()
+  f()
+
+proc main =
+  var y = [1, 2, 3, 4]
+  y = default(array[4, int])
+  for a in y: doAssert(a == 0)
+
+  var x = 4
+  x = default(int)
+  echo x
+
+main()